Normal adult hemoglobin comprises four globin proteins, two of which are alpha (α) proteins and two of which are beta (β) proteins. During mammalian fetal development, particularly in humans, the fetus produces fetal hemoglobin, which comprises two gamma (γ)-globin proteins instead of the two β-globin proteins. During the neonatal period, a globin switch occurs, referred to as the “fetal switch”, at which point, erythroid precursors switch from making predominantly γ-globin to making predominantly β-globin. The developmental switch from production of predominantly fetal hemoglobin or HbF (α2γ2) to production of adult hemoglobin or HbA (α2β2) begins at about 28 to 34 weeks of gestation and continues shortly after birth until HbA becomes predominant. This switch results primarily from decreased transcription of the gamma-globin genes and increased transcription of beta-globin genes. On average, the blood of a normal adult contains less than 1% HbF, though residual HbF levels have a variance of over 20 fold in healthy adults and are genetically controlled.
Hemoglobinopathies encompass a number of anemias of genetic origin in which there is a decreased production and/or increased destruction (hemolysis) of red blood cells (RBCs). These also include genetic defects that result in the production of abnormal hemoglobins with a concomitant impaired ability to maintain oxygen concentration. Some such disorders involve the failure to produce normal β-globin in sufficient amounts, while others involve the failure to produce normal β-globin entirely. These disorders associated with the β-globin protein are referred to generally as β-hemoglobinopathies. For example, β-thalassemias result from a partial or complete defect in the expression of the β-globin gene, leading to deficient or absent HbA. Sickle cell anemia results from a point mutation in the β-globin structural gene, leading to the production of an abnormal (sickle) hemoglobin (HbS). HbS is prone to polymerization, particularly under deoxygenated conditions. HbS RBCs are more fragile than normal RBCs and undergo hemolysis more readily, leading eventually to anemia.
Recently, the search for treatment aimed at reduction of globin chain imbalance or predisposition to hemoglobin polymerization in patients with β-hemoglobinopathies has focused on the pharmacologic manipulation of fetal hemoglobin (α2γ2; HbF). The therapeutic potential of such approaches is suggested by observations of the mild phenotype of individuals with co-inheritance of both homozygous β-thalassemia and hereditary persistence of fetal hemoglobin (HPFH), as well as by those patients with homozygous β-thalassemia who synthesize no adult hemoglobin, but in whom a reduced requirement for transfusions is observed in the presence of increased concentrations of fetal hemoglobin. Furthermore, it has been observed that certain populations of adult patients with β chain abnormalities have higher than normal levels of fetal hemoglobin (HbF), and have been observed to have a milder clinical course of disease than patients with normal adult levels of HbF. For example, a group of Saudi Arabian sickle-cell anemia patients who express 20-30% HbF have only mild clinical manifestations of the disease. It is now accepted that hemoglobin disorders, such as sickle cell anemia and the β-thalassemias, are ameliorated by increased HbF production.
As mentioned earlier, the switch from fetal hemoglobin to adult hemoglobin (α2γ2; HbA) usually proceeds within six months after parturition. However, in the majority of patients with β-hemoglobinopathies, the upstream γ globin genes are intact and fully functional, so that if these genes become reactivated, functional hemoglobin synthesis could be maintained during adulthood, and thus ameliorate disease severity. Unfortunately, the in vivo molecular mechanisms underlying the globin switch are not well understood.
Evidence supporting the feasibility of reactivation of fetal hemoglobin production comes from experiments in which it was shown that peripheral blood, containing clonogenic cells, when given the appropriate combination of growth factors, produce erythroid colonies and bursts in semisolid culture. Individual cells in such colonies can accumulate fetal hemoglobin (HbF), adult hemoglobin (HbA) or a combination of both. In cultures from adult blood, nucleated red cells accumulate either HbA (F-A+) only, or a combination of HbF and HbA (F+A+). Importantly, individual colonies contain both F+ and F− cells, indicating that both types are progeny from the same circulating stem cells. Thus, during the early stages of development in culture, cells execute an option, through currently unknown mechanisms, whether or not to express HbF. The proportion of adult F+ cells developing in culture does not appear to be preprogrammed in vivo, but appears to depend on culture conditions: A shift into the combined HbF and HbA expression pathway can, for example, be achieved in vitro by high serum concentrations, due to the activity of an unidentified compound that can be absorbed on activated charcoal.
Overall, identification of molecules that play a role in the globin switch is important for the development of novel therapeutic strategies that interfere with adult hemoglobin and induce fetal hemoglobin synthesis. Such molecules would provide new targets for the development of therapeutic interventions for a variety of hemoglobinopathies in which reactivation of fetal hemoglobin synthesis would significantly ameliorate disease severity and morbidity.